This invention relates to laser devices that produce optical energy of tightly controlled optical frequency, particularly for use in telecommunications applications. More particularly, the invention relates to devices that produce a specified optical frequency independent of thermal variations, while possessing the ability to be tuned or switched among alternative optical frequencies by thermal, electric field, or other control means without modifying the specified frequencies.
The growth of demand for subscriber bandwidth has led to great pressure to expand the capacity of the telecommunications networks. Dense wavelength division multiplexing (DWDM) allows high bandwidth use of existing fiber, but low-cost cost components are required to enable provision of high bandwidth to a broad range of customers. Key components include the source, the detector, and routing components, but these components should preferably be addressable to any of the frequency channels. These channels are currently defined by the ITU as xcexdn=xcexd0xc2x1n dxcexd, where xcexd0 is the central optical frequency 193.1 THz and dxcexd is the specified frequency channel spacing that may equal a multiple of 100 GHz or 50 GHz. Systems have also been demonstrated based on other fixed spacings, and based on nonuniform frequency spacings.
Semiconductor lasers with built-in gratings such as DFB and DBR lasers are currently used to produce the frequency-specific lasers needed to transmit over optical fibers. However, current fabrication techniques do not allow high yield production of a given frequency channel because of index of refraction variations in the InP-based materials. Because silica, polymer, and other optical materials offer greater stability of index of refraction, many types of hybrid lasers have been tested in which a semiconductor gain medium is combined with a grating fabricated in another material. Single frequency hybrid waveguide lasers have been demonstrated with semiconductor waveguide amplifiers to obtain the benefits of frequency selectivity and tunability. See for example * J. M. Hammer et al., Appl. Phys. Lett. 47 183, (1985), who used a grating in an external planar waveguide, by * E. Brinkmeyer et al., Elect. Lett 22 134 (1986) and * E. I. Gordon, U.S. Pat. No. 4,786,132, Nov. 22, 1988 and * R. C. Alferness, U.S. Pat. No. 4,955,028, Sep. 4,1990, who used a grating in a fiber waveguide, by * D. M. Bird et al., Elect. Lett. 27 1116 (1991) who used a UV-induced grating, by * W. Morey, U.S. Pat. No. 5,042,898, Aug. 27, 1991 who used a fiber grating with thermally compensated package, by * P. A. Morton et al., Appl. Phys. Lett. 64 2634 (1994) who used a chirped grating, by * D. A. G. Deacon, U.S. Pat. No. 5,504,772, Apr. 2, 1996, who used multiple gratings with optical switches, by * J. M. Chwalek, U.S. Pat. No. 5,418,802, May 23, 1995, who used an electro-optic waveguide grating, by * R. J. Campbell et al., Elect. Lett. 32 119 (1996) who used an angled semiconductor diode waveguide, by * T. Tanaka et al, Elect. Lett. 32 1202 (1996) who used flip-chip bonding, and by * J-M. Verdiell, U.S. Pat. No. 5,870,417, Feb. 9, 1999, who adjust for single mode operation. Single frequency hybrid waveguide lasers have also been demonstrated with fiber waveguide amplifiers. See * D. Huber, U.S. Pat. No. 5,134,620, Jul. 28, 1992 and * F. Leonberger, U.S. Pat. No. 5,317,576, May 31, 1994.
Many robust thermo-optic materials are available today including glass and polymer materials systems that can also be used in fabricating waveguide optical components. See * M. Haruna et al., IEE Proceedings 131H 322 (1984), and * N. B. J. Diemeer, et al., J. Light. Technology, 7, 449-453 (1989). Recently, thermally tunable gratings have been fabricated in polymer waveguides and resonators. See * L. Eldada et al., Proceedings of the Optical Fiber Communications Conference, Optical Society of America, p. 98 (1999), and * N. Bouadma, U.S. Pat. No. 5,732,102, Mar. 24, 1998.
Thermal compensation of laser resonators is a requirement in components that must operate robustly within the narrow absolute frequency bands of the DWDM specifications. Thermally compensated resonators have has been shown using polymer materials. See * K. Tada et al., Optical and Quantum Electronics 16, 463 (1984). Thermally compensated packages for fiber grating based devices have also been shown. See * W. Morey, U.S. Pat. No. 5,042,898, Aug. 27, 1991, * G. W. Yoffe et al, Appl. Opt. 34 6859 (1995), and * J-M. Verdiell, U.S. Pat. No. 5,870,417, Feb. 9, 1999. Thermally compensated waveguides using mixed silica-polymer materials have also been shown to produce temperature independent characteristics. See * Y. Kokubun et al., IEEE Photon. Techn. Lett. 5 1297 (1993), and * D. Bosc, U.S. Pat. No. 5,857,039, Jan. 5, 1999. Silica-polymer waveguides have also been used for interconnecting laser devices. See * K. Furuya U.S. Pat. No. 4,582,390, Apr. 15, 1986.
The grating assisted coupler is a useful device for frequency control. Grating assisted couplers as described in * R. C. Alferness, U.S. Pat. No. 4,737,007, Apr. 12, 1988, are known in many configurations including with mode lockers, amplifiers, modulators, and switches. See * A. S. Kewitsch, U.S. Pat. No. 5,875,272, Feb. 23, 1999. Grating assisted couplers have been used in resonators including lasers, mode lockers, etalons, add-drop filters, frequency doublers, etc. See for example * E. Snitzer, U.S. Pat. No. 5,459,801, Jan. 19, 1994, and * D. A. G. Deacon, U.S. Pat. No. 5,581,642, Dec. 3, 1996.
What is needed is a laser that operates robustly at a frequency specified for DWDM systems, with the operating frequency independent of environmental variations such as temperature and humidity. Ideally, this laser should also be tunable among many or all of the DWDM channels, and it should be inexpensive and easy to produce and test.
According to the invention, an amplifier device is combined with a material with negative index of refraction dependence on temperature to produce a laser device with cavity length and index of refraction control to accomplish temperature independent coincidence between cavity modes and a set of specified frequencies such as the DWDM optical channels in telecommunications applications. The free spectral range may be adjusted to equal a rational fraction of a specified frequency interval. The operating frequency may be defined by a frequency selective feedback element that is thermo-optically tuned by the application of heat from an actuator without substantially tuning the cavity modes. The operating frequency may be unique and it may be induced to hop digitally between the specified frequencies. In a particular embodiment, semiconductor amplifier and polymer waveguide segments form a linear resonator with a thermo-optically tuned grating reflector. In a further embodiment, an amplifier and two waveguides from a tunable grating assisted coupler form a ring resonator. Tuning may also be accomplished by means of applying an electric field across a liquid crystal portion of the waveguide structure within the grating. Methods are described of bringing the free spectral range of the cavity within tolerance, including intracavity methods of ablating material, depositing material, and exposing material to radiation.
The advantages of the invention include the fact that it provides a robust, athermal set of operating frequencies tied to a specified set of optical frequency channels. Digital tuning may be provided among these channels by thermal or other means without substantially modifying the specified frequency channels so that error-free channel selection is enabled among the provided channels. No wavelockers or other means are needed to specify the channels of operation. A simple method of channel selection is available, and direct modulation of the amplifier medium is available for data transmission, avoiding the need for a modulator.